1. Field of the Invention
The present invention relates to a developing unit of a liquid electrophotographic printer, and more particularly, to a developing unit of a liquid electrophotographic printer having an improved structure for cleaning the surface of a squeegee roller.
2. Description of the Related Art
A typical liquid electrophotographic printer such as a color laser printer includes a developing unit 10 for developing an electrostatic latent image formed on a photoreceptor web 1 by means of light scanning by a laser scanner to form an image of a predetermined color, as shown in FIG. 1. The developing unit 10 includes a developing roller 11 for developing the electrostatic latent image by forming a development gap G with the photoreceptor web 1 which is about 150 xcexcm and filled with developer, a squeegee roller 12 installed at the rear end of the developing roller 11 for pressing the photoreceptor web 1 to squeegee surplus developer after development, and a manifold 15 installed to enclose part of the outer circumferential surface of the developing roller 11 for guiding flow of the developer so that the developer pumped from a developing tank 20 is continuously provided to the development gap G. Reference numeral 13 denotes a brush roller for brushing the developer flowing down along the outer circumferential surface of the developing roller 11 to prevent it from entering the development gap G again. Reference numeral 16 denotes a blade installed to contact the developing roller 11 to perform the same function. Reference numeral 14 denotes a blade for squeegee roller 12. The squeegee roller blade 14 is off the squeegee roller 12 during development but contacts the squeegee roller 12 during a drip-line removing mode after the development is completed. A drip-line removing mode is a mode in which the squeegee roller 12 is rotated in a direction opposite to the direction in which the photoreceptor web 1 circulates so as to remove the developer adhering to the photoreceptor web 1 between the squeegee roller 12 and the developing roller 11. Reference numerals 11a and 12a denote a development backup roller and a squeegee backup roller, respectively.
When development is carried out in the above structure, a voltage of about +600 V is applied to the photoreceptor web 1. The voltage in an area of the photoreceptor web 1 where the electrostatic latent image is formed by light scanning of the laser scanner 30 drops to about +100 V. A voltage of about +400 V is applied to the developing roller 11. Since toner particles distributed in solvent of developer are charged positively, the toner particles in the developer which is provided to the development gap G adhere to the electrostatic latent image having a relatively low voltage. That is, the electrostatic latent image is developed. The surplus developer is removed by the squeegee roller 12.
When the development is repeated, toner particles of the developer adhere to the squeegee roller 12. The squeegee roller 12 squeegees surplus developer and concurrently presses the developed image to make it filmy. Here, some toner particles on the developed image adhere to the squeegee roller 12. The toner particles adhering to the squeegee roller 12 are transferred to the next image, such that the subsequent images may be continuously contaminated. In particular, in the case of a color laser printer for developing and printing a color image, since four colors are developed by four developing units in order to make a color image, if toner particles adhering to the squeegee roller 12 are mixed, an image of a desired color cannot be realized. For example, when four developing units are arranged to develop four colors, that is, yellow, cyan, magenta and black, it is assumed that the toner image developed by the first yellow developing unit passes a squeegee roller of the second cyan developing unit. Here, the toner particles of a cyan color developed on the photoreceptor web 1 are pressed to adhere to the squeegee roller of the cyan developing unit, that is, a reverse transfer occurs. The cyan toner particles adhere to the subsequent image passing the yellow developing unit so that the image is contaminated. Such a phenomenon is repeatedly generated on the surface of an image of the photoreceptor web 1 corresponding to the circumference of the squeegee roller, which is referred to as a squeegee offset phenomenon.
The proceeding status of the squeegee offset phenomenon is shown in FIGS. 2 through 4. That is, as shown in FIG. 2, when toner images C and Y developed by a cyan developing unit and a precedent yellow developing unit pass the squeegee roller 12 of the, cyan developing unit, the cyan image C is pressed first by the squeegee roller 12, as shown in FIG. 3. Here, some toner Cxe2x80x2 of the image is reverse transferred to the squeegee roller 12 and adheres to the subsequent yellow image Y, as shown in FIG. 4. Such a phenomenon becomes serious in the subsequent image among the four developing units since the squeegee offset phenomena at the precedent units are overlapped continuously. Thus, the above phenomenon is a hindrance to a high quality image and further causes contamination of developer in the subsequent developing unit.
To overcome the above problem, a means for cleaning the surface of the squeegee roller 12 with only a mechanical force utilizing a frictional contact has been suggested. However, the load to the rotating squeegee roller greatly increases. Since the squeegee roller 12 is driven by a driving force of the photoreceptor web 1, if the squeegee roller 12 is not properly rotated, an image is deteriorated. Also, when the squeegee roller 12 is formed of a material having a low surface energy, that is, a low frictional force, slippage is generated between the squeegee roller 12 and the photoreceptor web 1 so that an image is deteriorated.
Therefore, a plan is needed to effectively handle toner particles which are reversely transferred to the surface of the squeegee roller 12.
To solve the above problems, it is an objective of the present invention to provide a developing unit of a liquid electrophotographic printer which is improved to effectively remove toner particles of developer adhering to the surface of the squeegee roller.
Accordingly, to achieve the above objective, there is provided a developing unit of a liquid electrophotographic printer which comprises a developing roller for forming a development gap with a photoreceptor web where developer is filled and developing an electrostatic latent image formed on the photoreceptor web, a squeegee roller installed at the rear end of the developing roller for closely pressing the photoreceptor web to squeegee surplus developer, and a cleaning mechanism for removing toner particles of the developer adhering to a surface of the squeegee roller using an electrical force.
It is preferred in the present invention that the cleaning mechanism comprises a cleaning roller installed to maintain a predetermined gap with the squeegee roller, a first voltage applying portion for applying a voltage to the squeegee roller; and a second voltage applying portion for applying a voltage lower than that of the squeegee roller to the cleaning roller, wherein the toner particles adhering to the squeegee roller move to the cleaning roller via a developer film formed at the gap due to a difference in voltage.